Overhead cranes which travel on wheels along spaced apart, generally parallel rails, are subject to the continuous problem of crane wheel wear and failure. In such overhead cranes, wheels roll along a rail surface such that a portion of a crane wheel comes into contact with the rail surface thereby subjecting that portion of the crane wheel to wear.
A typical prior art crane wheel 20 is shown in FIGS. 1 and 2. The crane wheel 20 includes a hub 22 which surrounds an axis of rotation 24 of the crane wheel 20. The hub 22 is part of a radially inner portion 26 which consists of a body or core material 28 of the wheel 20. The crane wheel 20 further includes a radially outer portion 30 which includes a working tread surface 32 and opposing outer flanges 34, 36 which have respective inner surfaces 38, 40. The working tread surface 32 and at least portions of the flange inner surfaces 38, 40 make up a wear area 42 of the crane wheel 20.
As can be appreciated by those skilled in the art, certain portions of a crane wheel need different physical characteristics as compared to other portions of the crane wheel. The different physical properties are necessary because of the different conditions encountered by the different parts of the crane wheel as the crane wheel is in service. The wear area that engages a rail of an overhead crane must be resistant to wear. Thus, this portion of the wheel should be hardened. The hub of the wheel may be machined after heat treating of the wheel for the reception of an axle and of various bearing members in a crane assembly. Thus, this portion of the wheel should preferably remain machinable after heat treating of the wheel. As a result, for these types of wheels, processes have been used in an attempt to harden areas subjected to wear while attempting to maintain other areas of the wheel ductile or, as-forged.
Two prior processes used to harden wear surfaces of a crane wheel and which are capable of providing the necessary surface hardness required to support and guide heavy crane wheel loads, are generally known as the salt bath process and the gas carburizing process.
The salt bath process involves heating the surface temperature of a crane wheel to roughly about 1650.degree. F. by immersing the entire wheel or part of the wheel into a molten salt bath. When immersing only part of the wheel at any given time, the wheel is usually mounted on a rotating member such that the flanges, working tread surface and part of the body come into contact with the salt bath as the wheel is rotated. The heating process takes from one to three hours depending on the size of the crane wheel. Once the desired temperature is reached, the wheel is removed from the molten salt bath and transported to a quench bath where the wheel may be spin quenched in a manner similar to heating the wheel as outlined above. Alternatively, the entire wheel may be submerged in the quench bath.
The gas carburizing process involves securing a crane wheel in place in a gas tight box. Air in the box is evacuated and replaced with a carbon rich gas. The box is then heated to roughly about 1650.degree. F. for six to 36 hours, depending on the size of the wheel and the desired case depth. The elevated temperature allows the crane wheel surface to accept carbon from the gas. The wheel obtains a high carbon level on the outside surfaces, including the wear area, which surfaces can then be exposed to a thermal transformation process in order to obtain high surface hardness at the exposed surfaces.
FIGS. 1 and 2 represent prior art crane wheels created according to prior methods such as those just described. As can be observed from the shaded-in portions 39, of the crane wheels 20, the flanges 34, 36 are completely through hardened. As will be further explained below, these through-hardened portions are extremely brittle and subject to possible failure upon adverse impact during use.